Crystalline copolymers, such as linear low density polyethylene (LLDPE) and ethylene vinyl acetate (EVA), are known to have molecular weight distributions and composition distributions. The properties of copolymers having similar average compositions can vary considerably depending upon the compositional distribution of the copolymer. For example, in co-pending application serial number 944,385 filed 12/19/B6 which is incorporated herein by reference, it was established that the molecular weight distribution for Exxon LL3001 linear low density polyethylene resin was narrower than the molecular weight distribution of another commercially available linear low density polyethylene. Compositional distributions are known to have a strong effect on the physical properties of copolymers, e.g., heat sealing, tear strength, and FDA extraction limits.
The molecular weight and compositional distributions of copolymers have been determined by solvent fractionation of the crystalline copolymer and analysis of the fractions for molecular weight and composition. Apparatus for solvent fractionating of copolymers are known in the art. Such apparatus typically operate by dissolving the copolymer in a hot solvent and allowing the solvent to cool to ambient temperature within a packed column. The copolymer crystallizes on the column packing as the polymer cools. It is known in the art to cool the polymer to slightly above ambient temperature, about 30.degree. C, before starting fractionation with pure solvent.
Fractionation of the crystallized polymer is typically conducted with a single solvent that is passed through the column at increasing column temperatures beginning at the lowest temperature obtained during cooling of the copolymer. Less soluble fractions of the copolymer are dissolved and removed from the column as the column temperature rises such that the solvent effluent from the column containing dissolved copolymer can be collected in consecutive fractions which are identified by a starting and ending column temperature. These fractions are then analyzed for molecular weight distribution and monomer composition by conventional means.